Catalytic naphtha reforming is an established petroleum refinery process. It is used for improving the octane quality of hydrocarbon feeds in the naphtha boiling range. Generally, reforming refers to the total effect of molecular changes on a hydrocarbon feed, produced by a number of reactions. Typical reforming reactions include dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, dehydrocyclization of paraffins and olefins, isomerization of substituted aromatics, and hydrocracking of paraffins. Typical reforming catalysts are multifunctional catalysts having a hydrogenation-dehydrogenation component dispersed on a porous, inorganic oxide support. The support typically also contains an acid functionality, normally provided by the use of a halogen, to mediate the reforming reactions.
A reforming unit typically comprises a plurality of serially connected reactors with furnaces for supplying additional heat to the reaction stream as it passes from one reactor to the next in order to compensate for the heat utilized in the overall endothermic character of the process. Conventionally, reforming processes have been operated as semi-regenerative or cyclic processes using fixed bed reactors or as continuous processes such as UOP CCR Platforming™ (Continuous Catalytic Regeneration Platforming™) using moving bed reactors. Proposals have been made for combining fixed and moving bed reactors using regenerators appropriate to the individual reactor types. Units of this hybrid type are disclosed, for example, in U.S. Pat. No. 5,190,638; U.S. Pat. No. 5,190,639; U.S. Pat. No. 5,196,110; U.S. Pat. No. 5,211,838; U.S. Pat. No. 5,221,463; U.S. Pat. No. 5,354,451; U.S. Pat. No. 5,368,720 and U.S. Pat. No. 5,417,843. Similar hybrid reforming units using combinations of fixed bed and moving bed reactors are described in NPRA Paper No. AM-96-50 “IFP Solutions for Revamping Catalytic Reforming Units” (1996 NPRA Annual Meeting, 17-19 Mar. 1996). U.S. Pat. No. 4,498,973 describes a moving bed reforming unit in which two moving bed reactor stacks share a common regenerator. UOP has recently announced its CycleX™ Process for increased hydrogen production from a fixed bed reforming unit by the addition of a circulating catalyst reactor as the final reactor in the reactor sequence. This reactor is provided with its own heater and regenerator as an expansion of existing assets rather than as a substitution of them: NPRA Paper AM-03-93.
Whatever the configuration of the unit, however, fixed bed semi-regenerative, fixed bed cyclic, continuous or hybrid, a hydrogen recycle loop is provided in order to maintain an adequate volume of hydrogen at a suitable pressure in all the reactors. Although the reformer is a net producer of hydrogen, it is necessary to maintain a hydrogen:oil ratio within defined limits in order to minimize catalyst aging. The hydrogen which is has been generated or recycled is not pure and, in fact, typically contains significant quantities of light hydrocarbons which have not been completely removed in the separators which follow the reactor section. The term “light hydrocarbons” used herein means a hydrocarbon mixture comprised of hydrocarbon compounds of about 1 to about 5 carbon atoms in weight (i.e., C1 to C5 weight hydrocarbon compounds). The composition of the recycle gas, together with other process variables is a significant process variable. Reformer units are typically designed for a maximum feed rate, recycle gas rate, and recycle gas composition. Once these maximum design conditions are exceeded, the pressure drop in the system can exceed the ability of the recycle gas compressor to achieve an acceptable recycle rate without increasing system pressure. Increasing the pressure may not, however, be desirable in many cases as it will tend to reduce reformate and hydrogen yields, or the unit may already be operating near the maximum design pressure of the equipment.
We have now devised an improved configuration for catalytic naphtha reforming units which enables recycle gas compressor limitations to be overcome and which enables existing units to be operated at a capacity exceeding those imposed by compressor limitations and new units utilizing such embodiments may be operated at a greater nominal capacity.